In recent years, a liquid crystal display device is employed in products in various fields including AV (Audio and Visual) and OA (Office Automation). Low-end products are equipped with a passive matrix-type liquid crystal display device of TN (Twisted Nematic) or STN (Super Twisted Nematic), and high-end products are equipped with a liquid crystal display device adopting an active matrix driving method where TFT (Thin Film Transistor), which is a three-terminal non-linear element, is used as a switching element.
A liquid crystal display device adopting an active matrix driving method is superior in color reproducibility, thinness, lightness, and low power consumption to CRT (Cathode Ray Tube), so uses of such a liquid crystal display device rapidly spread. However, in the case where TFT is used as a switching element, in its producing process, it is necessary to repeat a thin film forming process and a photolithography process not less than 6 to 8 times, so the cost rises. For this reason, it is a most serious problem to lower the cost of production.
On the contrary, a liquid crystal display device where a two-terminal non-linear element is used as a switching element is superior in cost to a liquid crystal display device using TFT, and is also superior in display quality to a passive matrix-type liquid crystal display device. For this reason, a market for the liquid crystal display device using a two-terminal non-linear element is greatly expanded.
As shown in FIG. 11, a liquid crystal display device using a two-terminal non-linear element is composed of a display panel 112, a scanning electrode signal driver 113 for applying a fixed voltage to a scanning electrode line of the display panel 112 line-sequentially, a data electrode signal driver 114 for applying a fixed voltage to a data electrode line according to displayed information, and a control section 111 for transmitting a control signal respectively to the scanning electrode signal driver 113 and the data electrode signal driver 114 in order to display inputted information from an input signal line 115.
As shown in FIG. 12, the display panel 112 is arranged so that picture elements are placed in a matrix-like pattern, and each picture element is arranged so that a liquid crystal element 125 and a two-terminal non-linear element 126 are connected in series across each scanning electrode line (Y1 through Ym) and each data electrode line (X1 through Xn).
The scanning electrode signal driver 113 is composed of a liquid crystal driving power generating circuit, a shift register, an analog switch, etc., and the data electrode signal driver 114 is composed of a shift register, a latch circuit, an analog switch, etc. (not shown).
In the above arrangement, as shown in FIGS. 13(a) through 13(e), a fixed voltage (one of the six levels of liquid crystal driving voltages V0 through V5) is applied respectively from the scanning electrode signal driver 113 and the data electrode signal driver 114 to the scanning electrode lines (Y1 through Ym) and the data electrode lines (X1 through Xn) based upon a latch pulse (LP) of FIG. 13(a) and a switching signal (M) of FIG. 13(b). For example, in the case where voltages represented by waveforms in FIGS. 13(c) and 13(d) are applied to Y1 and X1, a voltage represented by a waveform in FIG. 13(e) is applied to both ends of a picture element connected to Y1 and X1. When a voltage represented by a solid line is applied, the liquid crystal element 125 is turned on, and when a voltage represented by a dotted line is applied, the liquid crystal element 125 is turned off.
As shown in FIG. 14, the two-terminal non-linear element 126 is characterized in that its equivalent resistance becomes smaller as the level of an applied voltage (V) becomes higher. Namely, as the level of the applied voltage becomes higher, the level of a current (I) becomes abruptly higher. A curved line 141 in the drawing shows an initial I-V characteristic, and when a voltage is continued to be applied, the I-V characteristic is shifted as shown by a curved line 142. The I-V characteristic is approximately symmetric with respect to the origin. Therefore, description of the case where a negative voltage is applied is omitted.
Since the two-terminal non-linear element 126 has the above I-V characteristic, a voltage applied to the picture element during selecting period (during period of display on picture elements) is held even during the non-selecting period. As a result, an active matrix-type liquid crystal display device using the two-terminal non-linear element 126 can be driven at higher duty compared to a simple matrix-type liquid crystal display device.
Furthermore, an active matrix-type liquid crystal display device can be driven by using a voltage averaging method for applying a voltage of FIG. 15 to a picture element like a simple matrix-type liquid crystal display device. In the voltage averaging method, in the case where the liquid crystal element 125 is turned on, a voltage represented by a solid line 151 is applied, and in the case where the liquid crystal element 125 is turned off, a voltage represented by a dotted line 152 is applied. In other words, the liquid crystal element 152 is turned on or off according to the level of the applied voltage during the selecting period. When a DC component is stored in the liquid crystal element 125, reliability is lowered. In order to avoid this, in general, alternating current is applied per frame (or per plural frames, or per plural lines) so that polarity of the applied voltage is reversed.
The active matrix-type liquid crystal display device using the two-terminal non-linear element 126 can realize high contrast and uniform display using the voltage averaging method.
However, in accordance with the above conventional arrangement, there arises a problem that a residual image (burning) is liable to be produced. For example, in a liquid crystal display device in normally white mode (in this mode, black is displayed when the liquid crystal element 125 is turned on), as shown in FIG. 16(a), a pattern composed of a white center portion P1 and a black peripheral portion P2 is displayed on the display panel 112, and the pattern is changed so that the whole screen becomes gray which is half tone. Then, as shown in FIG. 16(b), a part of the pattern which was previously displayed remains, so the whole screen does not become uniform. In other words, there is a difference in display between the white center portion P1 and the black peripheral portion P2, and thus a residual image is produced.
The residual image is caused by a shift in a voltage-dependent I-V characteristic in the two-terminal non-linear element 126. In other words, when the voltage is continued to be applied to the non-linear element 126, as mentioned above, the I-V characteristic is shifted from the curved line 141 to the curved line 142 (see FIG. 14). Accordingly, a T-V (transmittance-voltage) characteristic of the liquid crystal element 125 is also shifted from a curved line 171 to a curved line 172 as shown in FIG. 17. For example, a voltage whose transmittance is 50% is shifted from V.sub.50 to V.sub.50' in the drawing.
As shown in FIG. 18, a shift amount of the voltage .DELTA.V (=V.sub.50' -V.sub.50) changes according to voltage applying time. Moreover, when the level of the applied voltage becomes higher, a shift amount .DELTA.V becomes larger. In the drawing a curved line 181 shows a shift amount .DELTA.V when a higher voltage than a curved line 182 is applied.
As a result, when the pattern of FIG. 16(a) is displayed, a shift amount .DELTA.V of the peripheral portion P2 to which a higher voltage is applied is larger compared with the central portion P1. Then, when the pattern is changed so that the whole screen becomes grey which is half tone, namely, so that a voltage with the same level is respectively applied to the central portion P1 and the peripheral portion P2, the transmittance of the peripheral portion P2 becomes higher compared with the central portion P1 (FIG. 17). Therefore, the residual image is produced as shown in FIG. 16(b).
In order to suppress the production of such a residual image, in Japanese Examined Patent Publication No. 5-68712 (Tokukohei 5-68712), selecting period is divided into two, and adjustment charges, which makes it possible to ignore initial charge dependency of the non-linear element, are injected into an electro-optical element, such as a liquid crystal element, through a non-linear element during the first half of the period, and charges according to display data are injected into the electro-optical element through the non-linear element during the latter half of the period. As a result, an image is displayed without depending on previous display.
In addition, in Japanese Unexamined Patent Publication No. 5-323385/1993 (Tokukaiehei 5-323385), polarity of a voltage to be applied during the first half of the period is opposite to a polarity of a voltage to be applied according to the display data during the latter half of the period. A polarization amount of an MIM (metal-insulator metal) element as the non-linear element is made constant by sufficiently heightening the level of the voltage to be applied during the first half of the period so that the polarization amount does not depend on turning ON/OFF of the liquid crystal element. As a result, an image is displayed without depending on previous display.
However, the above driving method lowers production of a residual image, but it is difficult to use the driving method in the scanning electrode signal driver 113 and the data electrode signal driver 114 for driving picture element through the voltage averaging method.
In other words, as shown in FIGS. 19(a) through 19(e), in the case where a scanning electrode signal (FIG. 19(c)) and a data electrode signal (FIG. 19(d)) are created by making a selection from the liquid crystal driving voltages V0 through V5 according to a switching signal M, a driving voltage to be applied to a picture element becomes an ON voltage (shown by a solid line in FIG. 19(e)) or an OFF voltage (shown by a dotted line in FIG. 19(e)). For this reason, it is difficult to control a level of a driving voltage during the selecting period.
In addition, since the polarity of the scanning electrode signal with higher level and the data electrode signal with higher level is changed, crosstalk is liable to be generated during the non-selecting period.